#define  _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <math.h>
int main()
{
	double n = 0;
	scanf("%lf", &n);
	double Fn = (pow((1 + sqrt(5)) / 2, n) - pow((1 - sqrt(5)) / 2, n)) / sqrt(5);
	printf("%.2f", Fn);
	return 0;
}